Khan Academy offers a huge library of more than 4,300 educational videos that are accessible and meant for anyone. The nonprofit aims to change education by offering it for free.

Number to Know

700 million: On this date in 1981, a worldwide television audience of more than 700 million people watched the wedding of Charles, Prince of Wales, and Lady Diana Spencer at St Paul's Cathedral in London.

This Day in History

July 29, 2005: Astronomers announce their discovery of the dwarf planet Eris.